The endomembrane system, consisting of nuclear envelope, endoplasmic reticulum, Golgi apparatus, secretory vesicles and plasma membrane, functions in both synthesis and exocytosis of secretory products from eukaryotic cells. In the highly specialized mammary epithelial cell, exocytosis of milk constituents involves the net discharge of cell surface membrane. Since there is a considerable net synthesis of new surface membrane, the mammary epithelial cell affords a unique opportunity to study endomembrane formation, differentiation and flow. During past years of this project, endomembrane components have been isolated and characterized. During the coming year effort will be directed toward establishing the kinetics of turnover of individual proteins and groups of antigenically related proteins in components of the endomembrane system. The mechanism by which secretory vesicles interact with apical plasma membrane will be explored by in vitro synthesis of a major membrane coat-associated polypeptide which appears to be involbed in membrane recognition and interaction.